1. Field of the Invention
The present invention relates generally to an apparatus, method and system for thermal management of an electronic system having semiconductor devices, and, more particularly, to an apparatus, method and system for mechanical isolation combined with removal and dissipation of heat generated by a high clock frequency, high circuit density semiconductor devices.
2. Description of the Related Technology
Ever increasing market pressure for smaller, faster, and more sophisticated electronic products using integrated circuits has driven the electronics industry to develop integrated circuits which occupy less volume yet operate at heretofore unheard of clock frequencies employing incredible circuit densities. For example, two current production integrated circuits which serve as microprocessors are manufactured by the Intel Corporation called the PENTIUM (PENTIUM is the registered trademark of the Intel Corporation) and the PENTIUM II (PENTIUM II is the registered trademark of the Intel Corporation). The PENTIUM has over 3 million circuits in a single semiconductor die using 0.6 to 0.35 micron technology, operates at speeds ranging up to 266 MHz. The PENTIUM II has over 10 million circuits in a single semiconductor die and operates at speeds ranging up to 400 MHz, and is projected to soon exceed 500 MHz.
Because of the fragility of integrated circuit dice, and their susceptibility to environmental influences and mechanical trauma, individual or multiple integrated circuit dice have traditionally been enclosed in a protective "package" such as Pin Grid Array ("PGA") or Ball Grid Array ("BGA"). These packages may be made of plastic or ceramic materials, and provide electrical leads so that the enclosed die (or dice) may be electrically connected to a substrate, such as a printed circuit board ("PCB").
As the electronic products which utilize these increasingly powerful integrated circuits continue to shrink in size, such as laptop computers and other consumer, commercial, and military electronics, the space available for mounting the packaged integrated circuit die (or dice) is also reduced. Unfortunately, as integrated circuits grow in complexity and circuit density, the number of package leads needed to connect the packaged die (or dice) to the substrate also increases, thereby requiring more, not less, area to provide reliable electrical interconnections between the surface mount package to the substrate. Further, as the number of package leads increases, so does the capacitance, inductance and resistance of the package leads, which can degrade signal fidelity to and from the die (or dice).
In an effort to eliminate the above problems associated with modern packaging, some integrated circuit manufacturers have eliminated packages, and placed the unpackaged integrated circuit die (or dice) directly on the substrate. This practice of connecting unpackaged die (or dice) directly on a substrate is generically referred to as "chip-on-board" packaging.
An example of chip-on-board technology which is currently being manufactured and sold is the Intel Corporation's TCP PENTIUM.RTM.. The TCP PENTIUM ("TCP" stands for Tape Carrier Packaging) is a version of the PENTIUM in which the microprocessor integrated circuit die is an unpackaged die mounted face up on a PCB substrate and electrically connected to the PCB substrate using tape automated bonding technology. The PCB substrate also has numerous other integrated circuit packages directly connected to the substrate. When multiple dice are mounted on the same substrate, whether some or all are packaged or unpackaged, the combination is usually referred to as a multi chip module ("MCM").
Chip-on-board die leads may be electrically connected to the substrate face down using solder ball bonding (also known as "flip-chip") or in either a face down or face up arrangement using tape automated bonding ("TAB"). The exposed face of the die (i.e. the face opposite the face directly connected to the substrate) may be covered with a mechanically protective encapsulent.
The move to unpackaged chip-on-board technology has overcome some of the problems associated with higher clock speeds and circuit densities, but as is often the case, a successful solution to one problem often creates one or more new problems which must be addressed. One problem with unpackaged dice is that although advances in passivation allow unpackaged dice to withstand normal environmental influences better, unpackaged dice are still fragile and easily damaged by very minor external mechanical trauma, whether or not the dice are topped with an encapsulent. Although traditional component boards and MCM's (i.e. those having only packaged dice) have always been regarded and treated as delicate, this has usually been due to the risk of static electric discharge during handling which could damage the integrated circuits, not the mere accidental touching of a packaged die on a substrate board. An unpackaged die (or dice) with an encapsulent cap generally should not be subjected to more than 4.5 kilograms (9.9 pounds) of force on the center of the exposed face, however, lower forces may be damaging depending on the specific design parameters of a given die (or dice). A human hand in the mere act of touching an object, typically can and will exert forces greater than 4.5 kilograms.
Component boards and MCM's are usually fabricated at one location and then transported to either a component assembly location of either the original equipment manufacturer or a third party assembler. Sometimes, the component boards and/or MCM's are sold directly to end users who either need to repair or upgrade existing electronic products. This presents component manufacturers with the dilemma of shipping factory tested known good boards and MCM's having unpackaged dies, only to experience a higher than acceptable mortality rate in the course of normal shipping, and more often than not, normal handling by third party assemblers or end users.
Another problem with an unpackaged die (or dice) is related to the dissipation of waste heat generated by the die (or dice), also known as thermal management. As clock frequency and circuit density increase and die size decreases, the die power density and resulting production of waste heat also increase. As the quantity of waste heat increases, the effective steady state operating temperature of the die may also increase. If the steady state operating temperature of the die becomes greater than the maximum functional operating temperature of the die, the integrated circuit die may suffer degraded performance and/or experience logic errors. If the steady state operating temperature of the die becomes high enough, the die may experience errors in clock timing potentially causing the chip and/or system to lock-up. If the temperature becomes extremely high, the die may become permanently damaged and fail.
In addition to thermal performance degradation and/or damage, another problem of chip-on-board technology associated with increased waste heat is caused by the differences in the thermal coefficients of expansion ("TCE") between the die and the substrate, commonly referred to as TCE mismatch. Integrated circuit dice are composed of silicon whereas most substrates are composed of organic materials. The TCE of organic substrates are much greater than the TCE of silicon dice, therefore as temperature increases the organic substrates expand more than the silicon dice. Further, in a powered state, unpackaged dice conductively transfer most of their generated waste heat to the substrate. Therefore when an electronic product containing a chip-on-board die is turned on, the die temperature rises from the ambient temperature to the steady state operating temperature, which also raises the temperature of the organic substrate. Because of the TCE mismatch, the substrate expands more than the chip-on-board die. This condition results in a large mechanical stress being placed on the mechanically fragile die and the electrical connections to the substrate. Repeated power cycling can result in mechanical fatigue and eventual failure of die or the electrical connections, thereby destroying the use and/or value of the electronic product.
The present accepted solution for thermal management and TCE mismatch of unpackaged dice is to use the substrate, with or without thermal vias at the die attachment site, as a heat sink wherein the waste heat generated by the unpackaged die (or dice) is conductively transferred from the die to the substrate where the heat is both conductively transferred away from the die in the substrate and also convectively and radiantly transferred from the substrate to the ambient environment. If additional thermal enhancements are required, such as an externally attached heat sink, the heat sink is attached to the side of the substrate opposite the side where the unpackaged die is mounted. If an external heat sink is attached to the substrate, this provides an additional conductive path to transfer heat away from the die to the substrate, and then on to the external heat sink, where the heat is radiantly and convectively transferred to the ambient environment. Unfortunately, with the current trend of increasing power densities and consequent increasing waste heat generation of unpackaged die (or dice), these thermal management techniques are limited at best and more likely unacceptably inadequate.
Another problem associated with increasing clock speeds of semiconductor devices is that of radio frequency interference ("RFI"), also known as electromagnetic interference ("EMI"). Current production semiconductor dice are operating at speeds which are the same as radio frequencies used in wireless communications. For example, United States television channel 13 operates at approximately the 210 MHz frequency, while at the other end of the spectrum analog cellular telephones both receive and transmit at frequencies centered at approximately 880 MHz. Further, semiconductor devices can both emanate (transmit) and intercept (receive) electromagnetic fields at the operating (fundamental) frequency of the semiconductor device, as well as, at other (harmonic) frequencies greater than the operating frequency. Both emanation and interception of electromagnetic fields is often undesirable. Emanation of undesired electromagnetic fields can interfere with proper operation of nearby electrical devices or radio signal reception/transmission, whereas interception of strong radio signal transmissions (such as from a nearby cellular phone) could possibly cause a semiconductor device to malfunction and produce erroneous output. Additionally, the Federal Communications Commission ("FCC") has issued regulations which require that semiconductor devices and electronic systems not emanate radio frequencies above certain very low power levels (Part 15 of FCC Rules).
Another problem associated with the increasing clock speeds of semiconductor devices which cause more power dissipation in these devices is that of reduced battery life and increased noise due to the increased use of electric powered cooling fans. As more heat is generated by the semiconductor devices inside of an electronic product case, passive convective air flow through the case becomes insufficient to dissipate this heat. When passive convective air flow is not sufficient, one or more electric powered cooling fans must be used to provide active convective air flow through the electronic product case to adequately remove the increased heat loads being generated.
One solution to removing the increased heat loads is to continuously operate a cooling fan whenever the electronic product, such as a computer, is on. When the electronic product is operating off of an external power source, such as an AC wall outlet, the continuous operation of the fan presents the problem of the continuous semi-audible drone of the fan which can annoy an operator. When the electronic product is operating from an internal power source, such as a battery, the continuous operation of the fan presents the additional problem of reduced battery charge life due to the continuous power draw by the fan.
A solution to extend the battery charge life is to reduce the cooling fan power draw by intermittently operating the cooling fan, that is to say, by periodically turning the fan on and off by use of a timer and/or a thermostat located inside the case. This solution, however, suffers from two drawbacks. First, the constant cycling of the fan between the off state to the 100 percent on state can produce large temperature swings in or near the semiconductor devices. Second, the constant cycling of the fan is more annoying to the human operator because of the sudden source of a semi-audible sound when the fan is turned on as well as the sudden absence of this same background semi-audible sound when the fan is turned off.
What is needed is an apparatus, method and system to provide the necessary thermal management of high power density packaged or unpackaged dice during normal operation, which minimizes both emanation and interception of electromagnetic fields by packaged or unpackaged high frequency dice, and which also protects unpackaged dice from mechanical trauma during normal transportation, handling, installation, and operation, and which further extends battery charge life while minimizing any semi-audible sounds produced by the cooling fan during normal operations as well as when the fan is energized or de-energized.